1. Field of the Invention
The present invention relates to electronic circuits of various functionality including communication and signal processing circuits. More specifically, the invention relates to heterodyning transceivers that use a square wave local oscillator to mix a radio frequency signal down to a frequency band near DC.
2. Description of the Related Art
Electronic circuits having various functionalities, such as communication functionality, signal processing functionality, and others, employ the principal of heterodyning to translate the frequency content of a signal from a frequency band for which signal processing is difficult to a frequency band for which signal processing is simple. For example, the evolution of portable communication devices to miniscule sizes imposes a market requirement of tiny antennas which in turn requires higher frequencies for transmission and reception in a transceiver circuit. FIG. 1 is a frequency domain graph showing a typical signal frequency characteristic for a signal such as a high frequency communication signal. The communication signal has a relatively narrow bandwidth superimposed upon a high frequency carrier. In an illustrative example, a communication signal may have a bandwidth f2-f1 of merely 5 MHz centered at a 900 MHz signal. Heterodyne processing is used to shift a narrow band signal centered at a high frequency to a low frequency or vice versa for transmission.
Typical heterodyning circuits in transceiver front ends include analog mixers and multipliers that shift as shown by the frequency domain graphs in FIGS. 2A, 2B, and 2C, and a circuit diagram of an analog multiplier in FIG. 2D. The analog mixer or multiplier translates the frequency spectrum of a communication signal fc shown in FIG. 2A through the application of a local oscillator which oscillates at a frequency fLO. An analog multiplier 200 shown in FIG. 2D shifts the frequency domain content of the communication signal fc down to near DC, specifically to fcxe2x88x92fLO, as is shown in FIG. 2D. The mixing operation generates harmonics at frequencies fcxc2x1nfLO. The frequency region of interest is the component cos(xcfx89cxe2x88x92xcfx89LO)t, which is typically isolated using a filtering operation or circuit. A local oscillator in the form of a sine wave has a frequency domain characteristic of an impulse as shown in FIG. 2B for a sine wave local oscillator with harmonic indicator n equal to 1.
The multiplication process requires the amplitude of the local oscillator signal to be as large as possible, ideally to be as large as the signal, to avoid signal attenuation at the output. However, the circuits often saturate at signal levels below the ideal value and become non-linear, implying that the local oscillator signal amplitude needs to be precisely controlled and also that local oscillator linearity is important to the extent that harmonics of local oscillator do not mix down additional noise of the mixing devices. Another problem with sinewave oscillators is that the Lo signal is typically connected to a differential pair that operates in the saturation region and therefore produces a significant amount of noise.
One way to avoid noise problems and linearity problems in particular systems is to use a square wave local oscillator signal instead of a sine wave.
An analog multiplier or mixer that mixes a signal fc with a square wave local oscillator improves heterodyning operation of a circuit. In various square wave analog multiplier or mixer embodiments, heterodyning performance is improved in noise reduction, saturation performance, linearity, and other measures by adding a DC current path in parallel to a signal current path of the multiplier or mixer.
Practical conventional implementations of a square wave mixer suffer from one or more of the noise, saturation, and linearity problems. In addition to mixing of the signal of interest, a DC bias current is also mixed resulting in large transient signals, and complex mixer behavior. Mixer designs typically attempt to avoid transient signal generation by current steering, thereby dissipating excess signal power and attenuating the signal of interest, disadvantageously increasing noise.
In accordance with one aspect of the present invention, the source of the attenuation is identified and eliminated, overcoming the attenuation and reducing or eliminating noise.
In accordance with one embodiment of the present invention, an apparatus includes a circuit with a first square wave oscillator branch and a second square wave oscillator branch. Both square wave oscillator branches including a DC current path and a signal current path. The signal current path is driven by the square wave oscillator signal and the inverse of the square wave oscillator signal, respectively. The DC current paths in both the branches are biased such that at DC current, the current in the first square wave oscillator branch is equal to the current in the second square wave oscillator branch.